Synthesis of the ABC and IJ ring fragments of yessotoxin

Article abstract of DOI:10.1016/j.tetlet.2006.04.018

Synthesis of a 6/6/6 tricyclic ether system (3) corresponding to the ABC ring fragment of yessotoxin (1) has been achieved via coupling of a triflate and a 2-lithiofuran followed by intramolecular hetero-Michael addition. The IJ ring fragment (4) of 1 was

Full text of DOI:10.1016/j.tetlet.2006.04.018

Tetrahedron Letters 47 (2006) 3975–3978
Synthesis of the ABC and IJ ring fragments of yessotoxin
Tohru Oishi,* Miho Suzuki, Koji Watanabe and Michio Murata
Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
Received 13 March 2006; revised 5 April 2006; accepted 7 April 2006
Abstract—Synthesis of a 6/6/6 tricyclic ether system (3) corresponding to the ABC ring fragment of yessotoxin (1) has been achieved
via coupling of a triflate and a 2-lithiofuran followed by intramolecular hetero-Michael addition. The IJ ring fragment (4) of 1 was
readily synthesized via successive Sharpless epoxidation and 6-endo cyclization of the resulting vinyl epoxide.
Ó 2006 Elsevier Ltd. All rights reserved.
Yessotoxin (1)1 is a marine polyether toxin produced by
the dinoflagellate Protoceratium species (Fig. 1).2,3 Re-
cently, glycoside analogs of 1, protoceratins (2), which
show potent cytotoxicity against human tumor cell lines,
have been isolated from this organism.4 Because of their
biological activity, coupled with their fascinating arched
molecular structure, 1 and its analogs have attracted the
attention of synthetic chemists.5,6 During the course of
our synthetic studies of 1, we developed an efficient
method for convergent synthesis of polycyclic ethers
via a-cyano ethers.7 The methodology was successfully
applied to the convergent synthesis of the CDEF8 and
FGHI9 ring systems via two-ring construction of the
central DE and GH rings, respectively. Herein, we de-
scribe a stereocontrolled synthesis of the ABC (3) and
IJ (4) ring fragments (Fig. 1), which are pivotal inter-
mediates in the total synthesis of 1.
Although iterative syntheses of the ABC ring fragments
of 1 have already been reported,5d,g we developed a
novel strategy for synthesizing the 6/6/6 tricyclic system
via two-ring construction starting with a triflate 610 pre-
pared from 5,11 as shown in Scheme 1. Treatment of 6
with the 2-lithiofuran derivative 9, which is readily pre-
pared from 7,12 resulted in the formation of coupling
product 10 in moderate yield (45% for two steps), in
contrast to the corresponding coupling reactions
with alkynyllithiums (67–94%)13 or oxiranyllithiums
(90–98%),5d–f,14 but comparable to the reaction with
alkenyllithiums (15–55%).15 Removal of the EE group
Figure 1. Structures of yessotoxin (1), protoceratins (2), and the ABC (3) and IJ (4) ring fragments of yessotoxin.
Keywords: Yessotoxin; Ladder-shaped polyether; Alkylative coupling; 2-Lithiofuran; Intramolecular hetero-Michael addition; Reductive
etherification.
*
Corresponding author. Tel.: +81 6 6850 5775; fax: +81 6 6850 5785; e-mail: oishi@ch.wani.osaka-u.ac.jp
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.04.018

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T. Oishi et al. / Tetrahedron Letters 47 (2006) 3975–3978
Scheme 1. Reagents and conditions: (a) 2,6-lutidine, Tf2O, THF, À78 to À30 °C, 1 h, then TBSOTf, À78 to À10 °C, 1 h; (b) PPTS, ethyl vinyl ether,
CH2Cl2, rt, 1 h, 95%; (c) n-BuLi, HMPA, THF, À78 °C, 7 min; (d) HMPA, THF, À78 °C, 30 min, 45% (two steps from 5); (e) PPTS, t-BuOH/H2O
(3:1), rt, 13 h, 82%; (f) NaClO2, NaH2PO4, 2-methyl-2-butene, CHCl3/H2O/pH 4.0 buffer/t-BuOH (1:1:2:1), 40 °C, 7 h, 12 58%, 13 21%; (g) PPTS,
(MeO)3CH, CH2Cl2, rt, 8 h, 14 64%, recovery of 12 28%; (h) TBAF, THF, rt, 45 min, 44%; (i) 70% HF–pyridine, pyridine, THF, 59 °C, 8 h, 16 77%,
17 6%; (j) p-TsOHÆH2O, benzaldehyde dimethylacetal, CH2Cl2, rt, 3 h, 87%; (k) NaBH4, EtOH, À78 °C to rt, 25 min, 18 46%, 19 46%; (l) DMP,
pyridine, CH2Cl2, rt, 3 h; (m) NaBH4, EtOH, rt, 1 h, 18 50%, 19 50% (two steps); (n) NAPBr, NaHMDS, THF/DMF (2:1), rt, 3.5 h; (o) CSA,
MeOH/CH2Cl2 (1:1), rt, 2.5 h; (p) 2,6-lutidine, TBSOTf, CH2Cl2, rt, 99% (three steps); (q) Et3SiH, BF3ÆOEt2, CH2Cl2, À50 to À20 °C, 1 h; (r) 2,6-
lutidine, TBSOTf, CH2Cl2, 0 °C, 30 min, 84% (two steps); (s) K2OsO4, NMO, acetone/H2O (2:1), rt, 2.5 h; (t) NaIO4, THF/H2O (1:1), rt, 2 h, 82%
(two steps).
of 10 was successfully achieved by treating with PPTS in
t-BuOH and H2O (3:1) to afford furyl alcohol 11. De-
spite the considerable number of precedents for oxida-
tive ring expansion of furyl alcohols with VO(acac)2
and TBHP,12,16 the ring expansion of 11 was not an easy
task, giving hemiacetal 12 (40%) with concomitant
formation of its C1317 epimer 13 (13%) and byprod-
ucts corresponding to t-butylperoxy acetals (30%).
After considerable experimentation (oxidation with
MCPBA,18 NBS,19 or singlet oxygen20), we found that
oxidation with NaClO2 in pH 4.0 buffer and chloro-
form21 afforded 12 in better yield (58%). The hemiacetal
12, in a mixture with its C9 epimer (3:1), was converted
to methyl acetal 14 as a single isomer by treatment with
PPTS and methyl orthoformate in 64% yield, with
recovery of 12 (28%). Under the reaction conditions
for removal of the TBS group of 14 with TBAF, intra-
molecular hetero-Michael addition22 spontaneously
occurred in a stereoselective manner to form the tricyclic
1
system 15. However, H NMR analysis showed this to
be the undesirable cis-isomer. Meanwhile, removal of
the TBS group of 14 with HFÆPy yielded hydroxyenone
16. In contrast to the previous results, treatment of 16
with p-toluenesulfonic acid in dichloromethane at room
temperature afforded the desired compound 17 as a
single isomer without formation of spiroketals.23 The
diastereo-switchable hetero-Michael reaction of the
enone under basic or acidic conditions is noteworthy
from both the synthetic and mechanistic point of view.
Attempts to convert the cis-isomer 15 into the trans-
isomer 17 failed under acidic (p-TsOH) and basic
(DBU) conditions.

T. Oishi et al. / Tetrahedron Letters 47 (2006) 3975–3978
3977
Contrary to our expectations,5d the reduction of ketone
17 with NaBH4 gave a mixture of the desired alcohol 19
and its epimer 18 in a 1:1 ratio as a separable mixture
(the reaction did not proceed at À78 °C). The undesired
19 was recycled to form 18 via an oxidation and reduc-
tion sequence. Protecting group manipulation of 18, that
is, NAP24 ether formation at C12 and subsequent con-
version of the benzylidene acetal to a bis-TBS ether
(99%), followed by reduction of the methyl ketal with
Having synthesized the ABC ring system (3), we turned
our attention to the synthesis of the IJ ring fragment (4),
starting with the tetrahydropyran derivative 5,9 which is
a common intermediate with the A ring (Scheme 2).
Successive protecting group manipulation of the diol
5—that is, bis-benzyl ether formation (21), hydrolysis
of the benzylidene acetal (22), bis-TBS ether formation
and subsequent selective hydrolysis—furnished primary
alcohol 23. Swern oxidation of 23 followed by treatment
of the resulting aldehyde with (E)-1-lithio-1,3-butadiene
generated from the corresponding tributylstannane27
afforded a mixture of alcohol 24 and its epimer 25 in a
1:2 ratio, which was separated by silica gel chromatogra-
phy. The stereochemistry of 25 was unambiguously
determined by NOE experiments on acetonide 29
derived from 25. The undesired 24 was converted to
25 via an oxidation and Luche reduction sequence
(25:24 = 5:1). Removal of the TBS group of 25 and
subsequent Sharpless epoxidation using L-(+)-diethyl
tartrate yielded hydroxyl epoxide 27. Under the reaction
conditions described, partial cyclization occurred to give
an inseparable mixture of 27 and pyranopyran 28
(1.5:1), which was treated with PPTS to give 28 in
93% yield for two steps. Protection of the diol 28 as a
bis-TES ether, followed by hydroboration and Dess–
Martin oxidation, afforded the IJ ring fragment 4.28
25
Et3SiH in the presence of BF3ÆOEt2 afforded 20 as a
single isomer in 84% yield (partial removal of the TBS
group occurred under conditions of reductive etherifica-
tion to give a mixture of alcohols, which were again pro-
tected as TBS ethers). Finally, oxidative cleavage of the
terminal olefin provided the ABC ring fragment 3.26
In conclusion, we have achieved a stereocontrolled syn-
thesis of the ABC and IJ ring fragments of yessotoxin.
Further studies directed toward the total synthesis of
yessotoxin are currently in progress in our laboratory.
Acknowledgements
This work was supported by Grant-in-Aid for Scientific
Research (B) 15350024 from JSPS and Grant-in-Aid for
Scientific Research on Priority Areas 16073211 from
MEXT. K.W. is grateful to the 21st Century COE Pro-
gram for financial support.
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J = 2.5 Hz, H15), 7.82–7.79 (3H, m, NAP), 7.70 (1H, s,
NAP), 7.49–7.45 (2H, m, NAP), 7.39 (1H, dd, J = 8.5,
1.5 Hz, NAP), 4.74 (1H, d, J = 12.0 Hz, NAP), 4.58 (1H,
d, J = 12.0 Hz, NAP), 3.93 (1H, dd, J = 11.0, 5.0 Hz, H4),
3.82 (1H, td, J = 8.5, 4.5 Hz, H13), 3.52 (1H, d, J =
11.5 Hz, H2a), 3.40 (1H, d, J = 11.5 Hz, H2b), 3.28 (1H,
td, J = 10.0, 4.5 Hz, H12), 3.23 (1H, td, J = 10.0, 4.5 Hz,
H7), 3.15 (1H, td, J = 10.0, 4.5 Hz, H9), 2.98 (1H, td,
J = 10.5, 4.5 Hz, H10), 2.90 (1H, td, J = 10.5, 4.0 Hz, H6),
2.81 (1H, ddd, J = 16.0, 4.5, 2.5 Hz, H14a), 2.57 (1H, dt,
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2.5 Hz, H14b), 2.17 (1H, dt, J = 11.0, 4.0 Hz, H8eq), 2.02
(1H, dt, J = 11.5, 4.5 Hz, H5eq), 1.58 (1H, q, J = 11.5 Hz,
H5ax), 1.49 (1H, q, J = 11.0 Hz, H11ax), 1.33 (1H, q,
J = 11.5 Hz, H8ax), 0.98 (3H, s, Me), 0.84 (9H, s, TBS),
0.83 (9H, s, TBS), 0.03 (3H, s, TBS), 0.02 (3H, s, TBS),
À0.02 (6H, s, TBS); ESI MS 739 (M + Na+ + MeOH).
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(1H, d, J = 11.5 Hz, Bn), 4.53 (1H, d, J = 12.0 Hz, Bn),
4.50 (1H, d, J = 12.0 Hz, Bn), 4.34 (1H, d, J = 11.5 Hz,
Bn), 4.19 (1H, td, J = 10.0, 3.0 Hz, H30), 3.81 (1H, d,
J = 2.5 Hz, H32), 3.75 (1H, ddd, J = 10.0, 7.0, 2.5 Hz,
H37), 3.73 (1H, dd, J = 12.0, 4.5 Hz, H34), 3.69 (1H, dd,
J = 10.0, 2.0 Hz, H38), 3.58 (1H, dd, J = 10.0, 2.5 Hz,
H31), 3.52 (1H, dd, J = 10.0, 7.0 Hz, H38), 3.27 (1H, ddd,
J = 11.5, 10.0, 4.5 Hz, H36), 2.65 (1H, ddd, J = 16.0, 3.0,
1.5 Hz, H29), 2.37 (1H, ddd, J = 16.0, 10.0, 3.5 Hz, H29),
2.28 (1H, dt, J = 11.5, 4.5 Hz, H35eq), 1.50 (1H, dt,
J = 12.0, 11.5 Hz, H35ax), 1.13 (3H, s, 33-Me), 0.95 (3H,
t, J = 8.0 Hz, TES), 0.93 (3H, t, J = 8.0 Hz, TES), 0.61
(2H, q, J = 8.0 Hz, TES), 0.60 (2H, q, J = 8.0 Hz, TES).
ESI-MS 707 (M + Na+).
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